Significant mortality attends emergency surgery to control hemorrhagic lesions of the gastrointestinal tract. Vasopressin is the drug of choice for selective reduction in blood flows to splanchnic viscera. We have measured the effects of this agent in splanchnic viscera of animals. Several important questions remain. Intra-arterial vasopressin is said to be (1) more effective than when given intravenously. Its long half-life (24 minutes) suggests the effects should be equal. We wish to test this hypothesis in monkeys. Pilot studies suggest that it is a correct hypothesis. We will measure flows with flowmeters, as well as flow distribution with radioactive microspheres. Arteriovenous oxygen differences will be recorded to determine oxygen extraction, an index of effective blood flow to mucosa. (2) I.V. vasopressin is toxic to the canine myocardium at concentrations necessary to reduce splanchnic blood flows. We suspect a species-specific response and wish to test this hypothesis in monkeys. We will measure cardiac output, aortic pressure, pulse rate, cardiac rate and rhythm during administration of I.V. vasopressin, in a dose-dependent fashion, to include those concentration clinically recommended in man. Pilot studies indicate our hypothesis is correct. The monkey tolerates much higher concentrations of vasopressin than the dog. Cardiac output will be measured with flowmeter and thermodilution techniques. (3) Vasopressin is a constrictor in the canine hepatic artery. Constriction is characterized by autoregulatory escape, a unique observation concerning vasopressin. The importance of confirming this observation in subhuman primates relates to the use of vasopressin by the surgeon and radiologist to control bleeding from branches of the celiac axis in man. Reports of hepatic necrosis have questioned the effects of vasopressin inadvertently entering the human hepatic artery. We hypothesize that this fear is unfounded, based on the probable phenomenon of autoregulatory escape, which we anticipate characterizes the monkey hepatic artery and probably that of man.